The invention relates to a process for manufacturing ceramic metal composite bodies, to the ceramic metal composite bodies and to their use.
In many technological fields, for example, for tribological applications, highly wear resistant ceramic metal composite bodies with good frictional properties are required. Applications may be at room temperature as well as the high-temperature range. Such ceramic metal composite bodies are manufactured by means of a process in the case of which a liquid-phase infiltration of ceramic pre-bodies by a molten metal and an exchange reaction coupled therewith take place. The metallic constituents can be charged as an external molten metal as well as an internal molten metal, which is obtained even before the molten infiltration/exchange reaction (SI/AR), by the metal phase being present in the ceramic phase in a distributed manner.
Processes for manufacturing metal ceramic composite bodies by means of the infiltration of porous ceramic bodies have been described in the literature. U.S. Pat. No. 5,535,857 claims the manufacturing of a metal ceramic brake disk by way of the infiltration of a porous SiC pre-body. For this ceramic body, the SiC powder is pressed into the required shape and presintered so that open pore channels remain. The porous disk is then infiltrated by means of an aluminum alloy, whereby a metal-reinforced ceramic matrix is created. During infiltration, the metal carries out no reaction with the matrix so that the temperature stability of the material depends on the reinforcing matrix. In the case of an aluminum infiltration, this means that the application limit of the material is 400.degree. C.
Another process also describes the infiltration of a ceramic pre-body by means of aluminum (U.S. Pat. No. 4,988,645). In this case, the ceramic body is manufactured by way of an SHS reaction (SHS-reaction: Self-propagating high-temperature synthesis is the ignition of a reactive mixture), in which case the reaction maintains itself and supplies the desired ceramic matrix as a reaction product.
In U.S. Pat. No. 4,033,400, the infiltration of a porous ceramic body with a liquid metal is claimed, in which case the matrix consists of Si.sub.3 N.sub.4 and the metal is composed of an Al-alloy. In this case also, it is clearly important that no reaction is to take place between the matrix and the metal.
The firm Lanxide Technology also claims a number of materials which were produced by way of metal infiltration (for example, European Patent Documents EP-B-0 368 785 and EP-B-0 368 784). These patents essentially claim new process steps, as, for example, the targeted oxidation of the ceramic pre-body.
So far, all patents have had in common that no reaction infiltration has taken place. One exception, in this case, was Patent U.S. Pat. No. 4,585,618 in which the infiltrated metal (aluminum) reacts with the matrix.
It is an object of this invention to produce a reinforced TiB.sub.2 /Al.sub.2 O.sub.3 ceramic material for electrolysis cells. For this purpose, a TiO.sub.2 /B.sub.2 O.sub.3 /Al-mixture is infiltrated by aluminum. The infiltration time is 100 hours. The reaction product consists of TiB.sub.2 /Al.sub.2 O.sub.3 /Al, in which case, Al.sub.3 Ti is also found on the surface which is not desirable.
The manufacturing of ceramic metal composite bodies presents the problem of requiring high process temperatures. This is connected with high costs because the furnaces and reaction vessels are exposed to high wear. If inert components are charged, high temperature and long reaction times must absolutely be avoided because, under these conditions, they are subjected to a serious corrosion by the molten metal. High melting temperatures occur, for example, when the metals titanium, zirconium, boron, vanadium and chromium are used. When low-melting alloys or metals are used, the maximal application temperature of the composite body will be at the low melting temperature of the alloy.
In addition, the use of external molten phases results in various process-related problems which are caused by the fact that no stationary reaction conditions exist. The penetration of the molten metal from the outside begins an exchange reaction. As a result, a reaction front pushes through the ceramic pre-body. Because of the exchange reaction, the front of the molten phase builds up reaction products and loses reactive metals. Component-shape-dependent and size-dependent concentration gradients therefore extend through the composite material. These concentration gradients can be reduced only by a very long aging at high temperatures. The gradients of the composition also lead to discontinuous physical and mechanical characteristics. In addition to the non-uniform distribution of the phases, the gradients may also result in different stoichiometric compounds with different physical or chemical characteristics.
Another problem is clogging of the pores by reaction products, which prevents a further penetration of the molten metal. Particularly in the case of large components, the edge areas are exposed to the molten metal significantly longer, whereby much reaction product is deposited there, whereas the internal areas come only briefly in contact with the molten metal. A precise checking of the porosity and of the pore size of the ceramic pre-body must therefore take place. A pore radius which decreases radially from the outside toward the inside would be ideal.
Finally, the infiltration may take place incompletely as a function of the type and composition of the porous ceramic pre-body, of the molten metal or of the implementation of the process so that an interfering residual porosity remains in the composite body. This problem occurs particularly when an internal molten metal is used because no additional material can be supplied to the porous pre-body made of the metal and the ceramic material.
Reference is made at this point to the applicant's patent application, German Patent Document DE 197 06 926 and U.S. Provisional Application Serial No. 60/040,496 with the title "for Manufacturing Ceramic Metal Compound Bodies and Use Thereof," which was filed with the same priority date as the present patent application.
It is therefore another object of this invention to develop a process for manufacturing ceramic metal composite bodies in which the above-mentioned disadvantages are avoided.
According to the invention, this object is achieved by a process for manufacturing ceramic metal composite bodies which has the following process steps:
producing a reaction mass from 40 to 95% by volume ceramics fraction and from 5 to 60% by volume of at least two-high-temperature melting metals as well as optionally inert components, the composition of the metals being selected such that a low-melting eutectic phase of a reactive and less reactive metal is formed; PA1 heating the reaction mass under inert conditions to a temperature which permits the formation of a molten phase and maintaining the temperature until an exchange reaction has concluded.
In an embodiment of the invention the ceramics fraction of the reaction mass is used in bulk as a powder mixture or is used as a pressed part from a powder mixture; or as presintered porous mixed ceramics from a mixture of nitrides, carbides, silicides or borides of the elements silicon, titanium, vanadium, chromium, iron, aluminum, boron or magnesium.
According to the invention, it is provided that the ceramics fraction has an open porosity of from 5 to 70%.
It is within the scope of the invention that the metals in the reaction mass in the form of a metal alloy are used in bulk as metal powder; or are used as compressed-powder charges or as coarse pieces, which contain at least two of the elements silicon, tin, titanium, zirconium, aluminum, boron, indium, magnesium, calcium, vanadium, chromium, iron, cobalt, nickel, and copper.
Furthermore, it is preferred that the ceramics fraction of the reaction mass contains as inert components 20 to 80% by volume ceramic reinforcing components in the form of short fibers, fibrous tissue, fiber mesh, platelets or whiskers.
It is a further embodiment of the invention that the ceramic reinforcing components are present in the form of coated or uncoated carbon fibers or silicon carbide fibers.
It is advantageous for the production of the reaction mass to take place by means of a single-layer or multi-layer stacking of the components, a simple mixing of the components, by the pressing of a powder mixture of the compounds, or by means of a combination of these methods.
According to the invention, reaction mass is heated in a closed reaction vessel under argon, under vacuum or in air.
A slow heating rate of between 0.05 to 3.degree. K/min is also preferred with pressed powder mixtures from the ceramic or metal phase.
It is also essential that, in the case of pressed or bulk powder mixtures from the ceramic or metal phase, an external pressure be applied during the heating, the molten metal infiltration and the exchange reaction.
A ceramic metal composite body manufactured by means of a process according to the invention is also within the scope of the invention.
Finally, the use of ceramic metal bodies in tribo-systems is also the scope of the invention.
It is an advantage of the invention that high process temperatures can be avoided.
According to the invention, no pure metals are used as the metal component but an alloy of two high-temperature melting metals, which have low-melting eutectic phases. The composition of the metal alloy is selected such that a low-melting eutectic phase comprising of a reactive and of a less reactive metal is present. As a result of the preferred reaction of the more reactive metal, the composition of the molten metal alloy changes during the exchange reaction in favor of the less reactive metal. Since, as a result, the composition of the remaining molten metal alloy moves away from the eutectic composition, after the exchange reaction, only a small fraction of the eutectic phase with a low melting point remains as the molten metallic phase. The metallic phase consists predominantly of the less reactive metal with a high melting point.
The problem of the concentration gradients and of the clogging of the pores by reaction products can also be solved by the process according to the invention. The problems connected with external molten infiltration can be avoided by the use of an internal metal phase. For this purpose, the metal phase is distributed in the ceramic pre-body even before the annealing operation. Preferably, an un-fired body made of a metal ceramic powder mixture is produced for this purpose. During the annealing, the molten metal phase is already homogeneously distributed in the ceramics. Thus, the molten infiltration/exchange reaction takes place simultaneously in the whole ceramic pre-body. Macroscopic concentration gradients are therefore not formed.
Another advantage of this approach is the exact control of the stoichiometry in the composite body; for, precisely the used metal quantity is reacted. In the case of the external molten infiltration, a more or less large molten reservoir must be made available for complete infiltration. The reaction of this reservoir can be controlled only poorly by way of the reaction duration and the immersion depth of the specimen. Another complication occurs if the molten phase can selectively dissolve certain components out of the composite material.
Finally, the problem of residuals interfering with porosity because of incomplete infiltration can also be solved by means of the approach according to the invention. In the case of the process with an internal molten phase, the metal fraction can be increased so far that a continuous and coherent molten phase is formed during the annealing. As a result, the whole body becomes soft, can collapse in itself or completely melt open and displaces the previously included porosity. In the case of this approach, the faithful dimensioning would be lost.
The specimen can be compressed by supplying an external pressure before or during the exchange reaction. This process is particularly advantageous for the process variant with an internal molten phase. The amount of the metal phase is selected such that it corresponds at least to the total porosity of the un-fired body. Since the metal phase is present here in a finely distributed manner, a brief contact pressure at or above the melting temperature is sufficient for compressing the metal phase. Low pressures may be used because the pre-body is easily deformable as the result of the molten phase. Since the quantity of molten metal phase corresponds at least to the initial porosity, nearly dense bodies are always produced in this manner. For cost aspects, it is important that the exchange reaction can then also be carried out without external pressure.